related question: having switched to my back-up 650 , there is definitely a visible difference in the shade of the "paper" background between the two 650s. I have checked that they have same firmware & that adjust view is set to default on both.
Is it a "trick of the eye" because one is silver surround, the other is black, or do different batches of e-ink screens come out slightly different from with other. the silver surround 650 seems to have more of a creamy colour tink to its screen,